Measurement of bulk liquid stored light hydrocarbons

Paul Boughton

Rob de Nijs highlights the application issues around the servo and radar technology when measuring light hydrocarbons.

For the determination of the liquid level inside bulk storage tanks using automatic tank gauges many technologies are available. For accurate custody transfer and legal metrology approved product level measurement on shore storage tanksmainly servo or radar technology is used.

The choice between these two depends of course on the customer requirements derived from his intentions on operating the storage facility in the best way. Factors like accurate inventory measurementoptimisation of the process and service requirements of the instruments play an important role in this selection process.

To comply with installation standards and recognised installation practices like ISO and APIthe installation of the servo gauge on LPG/LNG tanks should be as Fig. 1.

On a standard used stilling wellmost of the times reduced at the top towards a

6-inch connectiona ball valve is used for process separation. This is required to take away the gauge without taking the tank out of operation. On top of thatthere is the so-called calibration chamber for inspection installation of the displacer.

Verifying the gauge without taking the tank out of operation should be possible. This is done with several gauge commands. Test is an action performed around the actual product surfacewhere LockBlock and Unlock can be used to lift the displacer to any desired locationhold it and lower it again.

Further there is the option of lowering the displacer towards the tank datum plate for bottom verification or lifting the displacer towards the tanktop position inside the calibration chamberdetermined at commissioning time. Inspection companies often use the top of the ball valve as reference position.

The total weight of the unwound measuring wire should be compensated for as well as for the mechanical expansion/contraction of the wire due to temperature differences.

The Enraf854ATG servo gauge is a solution for measuring LPG/LNG. Standard installation materials can be used as stilling wells and ball valves where there are no additional requirements to comply with international standards. The accuracy of the Enraf854 series servo gauges under reference conditions is ±0.4mm over the entire measuring range.

The total installed accuracy of the Enraf servo gauges in operation on an LNG/LPG application is measured to be within the ±1mmtaking practical application aspects into consideration.

Weights and Measures authorities recognise the performance of the Enraf854 series of servo gauges and approved them for use on legal metrology and custody transfer applicationsincluding measurement on LNG/LPG.


Radar technology

To comply with installation standards and recognised installation practices like ISO and APIthe installation of the radar gauge on LPG/LNG tanks should be as in Fig. 2. or 3.

To comply with the recommended installation practice of removing the gauge without taking the tank out of operationthe horn antenna is mostly seen as a fixed mechanical part of the tank structure. The 1-inch full bore ball valve in Fig. 2. leaves the antenna installed in an operational tank.

When the horn antenna is considered as part of the gaugethe 4-inch full bore ball valve separates the entire radar gauge from the process Fig. 3. The horn antenna is now installed on top of the ball valveusing an additional spool piece.

When using radar technology for level measurement in closed bulk storage tanks containing light hydrocarbonsthere are two issues to think about from a technical point of view:

  • The low dielectric constant difference between the vapour above the liquid and the liquid itself.
  • The effect the vapour has on the radar signal propagation speed.

Measuring products with low dielectrical constants like LPG/LNGthe signal strength reflected back from the product level is quite low. To ensure a reliable level reading the radar signal has to be focussed on a small product area in 4-inch stilling wells. As the diameter of the stilling well determines the radar propagation speedthe inner diameter should be constant over the entire measuring range.

To cope with the pressure inside the tank (LPG application)a 4-inch horn antenna is used to smoothly adjust the diameter of the feeder electronics towards the inner diameter of the stilling well.

The use of a full bore ball valve is required to guide the radar signal towards the product level without diameter changes and act as a process sealcomplying with international regulations. Inside the stilling wellverification pins are installed to be able to verify the performance of the gauge without taking the tank out of operation.

The LPG/LNG vapour above the liquid slows down the propagation speed of the radar signal. As every radar system is calibrated in air and the exact content of the vapour is hardly stablecompensation for this effect should be done using vapour pressure and temperature measurement.

The Weights and Measures institutes recognise the effect the vapour might have on the accuracy of the gauge. Therefore they have made a distinction in the range of products to be measured.

For petrochemical products having a molecular structure containing less than eight C-atomsthe effect of the accuracy has to be calculated and possibly compensated for. Forms with calculations should be submitted to the authorities to have their formal agreement for a linear compensation. Radar technology on LNG/LPG tanks is normally not approved by Weights and Measures due to the unpredictable vapour composition and the effect this has on the overall accuracy.

As with any other radar vendorEnraf incorporates radar gauge technology in the product portfolioas this offers a contact free measurement without moving parts. The Enraf SmartRadar series offer a solution for measuring LPG/LNGwhen the above mentioned installation aspects are taken into consideration.

The accuracy of the Enraf970 SmartRadar gauge under reference conditions is ±0.4mm over the entire measuring range. The total installed accuracy of the Enraf 970 SmartRadar in operation on an LNG/LPG application is measured to be within the ±1mmtaking practical application aspects into consideration and vapour composition have a linear effect on the radar propagation speedand thus can be compensated for.

Weights and Measures authorities recognise the performance of the Enraf970 SmartRadar gauge and approved it for use on legal metrology and custody transfer applications. Measurement on LNG/LPG is not approved due to the untracable vapour composition and absence of the possibility to verify with a manual handdip.

Measurement on LPG/LNG can be done with either servo or radar technology. As for the radar technology installation aspects have a crucial impact on the radar gauge performance. The W&M authorities do not allow radar technology for fiscal measurements on LNG/LPG. Enraf prefers to offer the servo gauging technology for the best level measurement performance and for easy of complying with international regulations.

Rob de Nijs is Product Manager InstrumentationEnraf Terminal AutomationDelftThe Netherlands. www.enraf.com

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